"I don’t understand,'' she said. "Like, why would anyone think this is a good idea?"

The ratio of fat to carbs and protein is 4:1 in the keto diet, with a goal of burning fat for energy rather than carbohydrates in order to lose weight. Michaels, 44, is not buying it.

"You know what we need to do? All fat and animal protein!’'' she said. "No! Bad plan. For a million reasons."

In the view of the former "Biggest Loser" trainer, the keto diet doesn't provide proper nutritional balance.

"Your cells, your macro molecules, are literally made up of protein, fat, carbohydrates, nucleic acids,'' she said. "When you do not eat one of the three macro nutrients — those three things I just mentioned — you’re starving yourselves.

"Those macro nutrients serve a very important purpose for your overall health and wellbeing. Each and every one of them."

A few studies have shown that the diet may promote weight loss, lower blood sugar and increase insulin sensitivity in diabetics while also boosting energy and mental focus. The downside is that it's hard to commit to the strict diet, and adjusting to such a small carbohydrate intake can cause micronutrient deficiency, fatigue, and nausea for a few weeks, Dr. Keri Glassman told TODAY in October.

There have been success stories, like Candace Brisco, a registered nurse from New Orleans who told TODAY last week about how the keto diet helped her shed 135 pounds. Virginia woman Tori Lewis spoke to TODAY in November that she dropped 120 pounds following the program.

TODAY's Savannah Guthrie and Al Roker have both tried the keto diet, with varying results.

Savannah spoke about her experience with Dr. Mehmet Oz on his show in November, saying she had less energy to work out due to the reduced carbohydrates but that her brain felt sharper throughout the day.

"In some ways you have to deprive yourself,'' Savannah said. "No carbs whatsoever, and that's really restrictive, but it's opened up some things that I would never let myself eat a lot of, such as cheese and bacon, so I like that."

Those restrictions are why Michaels would not recommend the keto diet to anyone.

"You don’t eat processed sugar, you don’t eat processed grains, and to make a very long story short: avoid the keto diet," she said. "Common sense. Balanced diet is key."